The childhood obesity epidemic raises important clinical and public health questions about the effects of both obesity and obesity treatment on bone development. Osteoporotic fractures are a major cause of morbidity and mortality in the aged. However, peak bone mass, achieved shortly after puberty, is the key determinant of lifetime osteoporotic fracture risks. The size and direction of effects of obesity and weight loss on bone health in childhood are unclear, partly because of bone ascertainment issues. Obese children and adolescents may have elevated fracture risks, contrary to expectations based on adult data. Primary aims of the proposed study are to: 1) characterize and compare bone health of obese and non-obese adolescents using the most accurate methods available, and 2) investigate the impact of comprehensive behavior weight control program on the bone health of obese adolescents. Aim 1 will use a case-control design to compare bone status measures of 88 obese adolescents (ages 10 to 14 years), to be recruited for a randomized obesity treatment trial, with the same measurements as a group of 51 contemporary non-obese control adolescents. For Aim 2, the 88 obese adolescents will be enrolled in a 12-month randomized trial, with 1:1 assignment, of a comprehensive, group, family-based, behavioral weight control program vs. individualized nutrition education (usual care). For both aims the primary outcome will be bone strength, estimated by stress-strength index, and measured by peripheral quantitative computerized tomography (pQCT) at the tibia, a weight-bearing site. Hypotheses are that bone strength for length will be higher in the obese compared to the non-obese adolescents (Aim 1), but will decrease with weight loss compared to usual care (Aim 2). Secondary aims are to assess the impact of obesity and weight loss on bone strength at the radius, on trabecular bone and potential mediators of the association between obesity and bone health, such as nutrition, physical activity, and muscle strength. Relevance to Public Health: Obesity and osteoporotic fractures are two significant and increasing public health problems. As it is unclear if adolescents who develop or are treated for obesity before they reach maximum bone mass are at increased or decreased risk for osteoporosis, this study will compare bone health of obese and non-obese adolescents and investigate the impact of a weight loss program on bone health.